Method of making a cadmium-telluride semi-conductive device



Feb. 4, 1958 D. DE NOBEL r I 2,822,299

METHOD OF MAKING A CADIUM-TELLURIDE SEMI-CONDUCTIVE DEVICE Filed Nov. 28, 1955 INVENTOR DIRK DE NOBEL AGENT United States Patent METHOD OF MAKING A CADMIUM-TELLURIDE SEMI-CONDUCTIVE DEVICE Dirk de Nobel, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application November 28, 1955, Serial No. 549,528

4 Claims. (Cl. 117-200) The invention relates to the manufacture of a semiconductive device, and, in particular, to the provision of a tellurium layer on a semi-conductive body of cadmiumtelluride.

Tellurium layers on cadmium-telluride, if this is of p-type conductivity, provide an ohmic contact, whereas on n-type conductive material they have a rectifying or a photo-sensitive efiect.

The invention is based on the recognition of the fact that in a simple manner a tellurium layer can be obtained on cadmium-telluride, by converting the cadmium-telluride superficially into tellurium.

According to the invention, the semi-conductive body is treated to this end with an oxidizing solution. If the solution for the treatment has too weak an oxidizing effect, no reaction takes place. In case of too strong an oxidizing effect, the cadmium-telluride is converted partly into tellurium oxide, cadmium being dissolved.

Under intermediate circumstances, the Cd of the Cd-Te is preferentially oxidized to CdO, which then dissolves in the solution so that a tellurium layer is produced on the cadmium-telluride body.

The oxidizing conditions may be intensified not only by the choice of the oxidizing substance, but also by the concentration of this substance in a solution, by the tem- 0on- Time inmin.

oxidizing Result means no reaction.

Te-layer.

Te-1ayer plus oxide Te-layer.

D0. D0. no reaction. Te-layer with oride spots. adhering black deposition with free Te. Te-layer.

a: new: we:

more

some sec.

FeOh.

no reaction. Te-layer. no reaction. Te-plus oxide spots. 'le-layer. black deposit. Te-layer plus some oxide.

Do. Te-layer. Te-la er plus oxi e spots. 'le-layer. re-layer plus oxide.

BNO:

lease Te-layer.

no reaction.

'le-layer.

Huwoeee we: was 5 ea 5 5 85 perature of the solution, and by the time during which the semi-conductive body is treated. Under strongly acidic conditions the reaction takes places at a lower temperature and the formation of tellurium oxide is less 5 troublesome. The conditions suitable for the oxidization may be fixed simply with the aid of a few experiments, as is indicated in the table, which shows the results of subjecting the Cd-Te body to different oxidizing treatments. The best treatments are obviously those that produce a tellurium layer only on the Cd-Te body.

For acidifying the solutions, use is preferably made of HCl, since acids such as H HNO and HClO, have an oxidizing eifect in themselves, and acids like HF, HBr and HI can be handled less readily. Two preferred examples follow, the second one to be described in connection with the accompanying drawing, of which Figs. 1 and 2 show operating characteristics of a Cd-Te semiconductor made by the invention:

Example 1 A plate of p-type conductive CdTe, specific resistivity about 1.5 X 10 ohm-cm. is treated for example with a solution of 0.3% of FeCl:.; at a temperature of 70 C. for five minutes. The tellurium layer produced superficially constitutes with the CdTe an ohmic contact.

Example 2 An n-type conductive CdTe crystal plate with about 3 X 10 charge carriers per cm. is provided with a tellurium layer by treating it at 40 with a solution of 20.2% of HCl and 3% of HNO for about one minute. The tellurium layer is removed on one side, for example by securing. On this side provision is made of an ohmic contact by fusing thereto indium at 500 C. in a mixture of nitrogen and 10% hydrogen. The rectifier obtained by means of the remaining tellurium layer has a currentvoltage characteristic curve as shown in Fig. 1.

Upon an exposure with about 2700 lux from a tungstentype lamp (colour temperature 2800" K.) the rectifier possesses a photo-E. M. F. and/or a photo-current. The short-circuiting current is pa. and the voltage at an infinite external resistance is 500 mv.

It is evident from Fig. 2, in which the variation of the photo-E. M. F. is plotted against the wavelength, that the maximum sensitivity lies at a wavelength of 8250 A.

What is claimed is:

1. A method of providing a tellurium layer on a cadminm-telluride semi-conductive body, which comprises subjecting a surface portion of said body to a preferential oxidizing treatment to convert the material of said surface portion into tellurium.

2. A method of providing a tellurium layer on' a cadmium-tellunide semi-conductive body, which comprises treating a surface portion of said body with an oxidizing solution of a concentration, at a temperature, and for a time suflicient to convert the material of said surface portion into tellurium.

3. A method as set forth in claim 2 wherein the solution contains a strong oxidizing agent in an acid medium.

4. A method as set forth in claim 3 wherein the acid medium is HCl.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,822,299 February 4, l958 Dirk de Nobel I It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, between lines 8 and 9, insert Claims priority, application Netherlands December 4, 1954.\

Signed and sealed this 22nd day of April 1958.

(SEAL) Attest:

KARL Ha AXLINE ROBERT C. WATSON Attesting Officer Conmissioner of Patents 

1. A METHOD OF PROVIDING A TELLURIUM LAYER ON A CADMIUM-TELLURIDE SEMI-CONDUCTIVE BODY, WHICH COMPRISES SUBJECTING A SURFACE PORTION OF SAID BODY TO A PERFERENTIAL OXIDIZING TREATMENT TO CONVERT THE MATERIAL OF SAID SURFACE PORTION INTO TELLURIUM. 